On 29 December 2013, ESA’s Mars Express will make the closest flyby yet of the Red Planet’s moon Phobos, skimming past only 45 km above its surface.

As the spacecraft passes close to Phobos, it will be pulled slightly off course by the moon’s gravity, by a few tens of centimeters. This small deviation will be measured using the spacecraft’s radio signals, and then translated into measurements of gravity, mass and density at different locations on the moon.

This animation shows how the flyby would appear from a vantage point near Mars Express, with Phobos passing by beneath. The relative movement between the spacecraft and moon has been speeded up ten times faster than actual.